What is Restoring force of a spring?

A glider of mass $m$ slides on a horizontal frictionless rail, tied to a spring of stiffness $k$ ($\mathrm{N}/\mathrm{m}$). Let $x$ be its displacement from equilibrium. Stretched or compressed, the spring pulls or pushes the glider back with the force $F = -kx$: proportional to the displacement and opposed to it — a restoring force again, now supplied by elasticity.